Cook chicken and cube; reserve liquid.,______,In a 3-quart saucepan melt butter; add onion and cook until tender but not browned.,Stir in flour, dry mustard, salt, thyme and pepper. Stir in milk and chicken broth all at once.,Cook and stir over medium heat until slightly thickened and bubbly.,Stir in vegetables, chicken and reserved liquid.,Cook and stir 4 to 6 minutes longer or until vegetables are tender and soup is heated through.